Negative electrode and battery

Abstract

A negative electrode 10 according to the present disclosure comprises a negative electrode mixture layer 11 containing a negative electrode active material and a conductive assistant. The negative electrode active material includes first active material particles containing a carbon material as a main component, and second active material particles containing silicon. The conductive assistant includes carbon nanotubes and carbon black. The carbon nanotube has an average fiber diameter of 0.5 nm or more and 6 nm or less, and an average fiber length of 1.2 μm or more and 8 μm or less. A mass ratio of the carbon black to the carbon nanotubes is 10 or more and less than 40.

Description

Anodes and Batteries

[0001] The present disclosure relates to anodes and batteries.

[0002] In recent years, secondary batteries such as lithium-ion batteries have been widely used in applications requiring high capacity, such as in-vehicle applications and power storage applications. The electrodes that constitute such batteries have a significant impact on battery performance. For this reason, various studies have been conducted on electrodes.

[0003] Higher capacity is required for secondary batteries such as lithium-ion batteries. Therefore, silicon (Si)-containing materials have been attracting attention as negative electrode active materials with high theoretical capacity densities. To achieve higher battery capacity, the current collection ability of the electrode during charging and discharging, i.e., the conductivity of the electrode, is also important.

[0004] For example, Patent Document 1 discloses a conductive composite containing silicon-containing inorganic compound particles that function as a negative electrode active material and a conductivity imparting agent that is an aggregate of carbon nanotubes, and proposes using this conductive composite in the negative electrode of a lithium ion secondary battery.

[0005] JP 2011-76948 A

[0006] As described above, Patent Document 1 proposes a conductive composite as a negative electrode, which includes a silicon-containing negative electrode active material and a conductivity imparting agent that is an aggregate of carbon nanotubes. In this conductive composite, the content of the conductivity imparting agent, i.e., the aggregate of carbon nanotubes, is high, ranging from 0.1 to 20 wt %. Thus, conventional negative electrodes containing carbon nanotubes contain a large amount of carbon nanotubes.

[0007] In addition, further increases in the amount of silicon-containing negative electrode active material are being considered with the aim of achieving higher capacity. Because silicon-containing negative electrode active materials undergo large volume changes during charge and discharge, increasing the amount of such negative electrode active material also requires increasing the amount of carbon nanotubes to maintain the current collection performance of the negative electrode. However, when forming a negative electrode by the commonly used method of applying a slurry in which various constituent materials are added to a dispersion medium and mixed, increasing the amount of carbon nanotubes to ensure sufficient current collection performance of the negative electrode increases the viscosity of the slurry, making it difficult to apply a uniform coating and obtain a good coating film. Furthermore, carbon nanotubes are relatively expensive, which increases manufacturing costs. On the other hand, because carbon nanotubes can achieve excellent current collection performance, reducing the amount makes it difficult to obtain good battery characteristics. The negative electrode with a high carbon nanotube content disclosed in Patent Document 1 is manufactured using a specialized and complicated manufacturing method, in which a composite in which the surfaces of active material particles are coated with carbon nanotube aggregates is first formed, and then the slurry is prepared.

[0008] The present disclosure provides a technology that enables the amount of carbon nanotubes in a negative electrode containing carbon nanotubes to be reduced while maintaining good battery characteristics, and also enables the viscosity increase of a slurry for forming a negative electrode prepared by a conventional method to be kept relatively small.

[0009] The negative electrode of the present disclosure comprises a negative electrode mixture layer containing a negative electrode active material and a conductive additive, wherein the negative electrode active material contains first active material particles containing a carbon material as a main component and second active material particles containing silicon, the conductive additive contains carbon nanotubes and carbon black, the carbon nanotubes have an average fiber diameter of 0.5 nm or more and 6 nm or less and an average fiber length of 1.2 μm or more and 8 μm or less, and the mass ratio of the carbon black to the carbon nanotubes is 10 or more and less than 40.

[0010] The technology disclosed herein makes it possible to reduce the amount of carbon nanotubes in a negative electrode containing carbon nanotubes while maintaining good battery characteristics, and also makes it possible to keep the increase in viscosity of the slurry used to form a negative electrode prepared by a conventional method relatively small.

[0011] Fig. 1 is a cross-sectional view showing a schematic configuration of an example of a negative electrode according to Embodiment 1. Fig. 2 is a longitudinal cross-sectional view showing a schematic configuration of an example of a battery according to Embodiment 2.

[0012] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0013] (Embodiment 1) Fig. 1 is a cross-sectional view showing a schematic configuration of an example of a negative electrode according to Embodiment 1. As shown in Fig. 1, a negative electrode 10 according to Embodiment 1 includes a negative electrode mixture layer 11. The negative electrode mixture layer 11 contains a negative electrode active material and a conductive additive. The negative electrode 10 further includes, for example, a negative electrode current collector 12, and the negative electrode mixture layer 11 is disposed on the negative electrode current collector 12.

[0014] The negative electrode active material includes first active material particles containing a carbon material as a main component and second active material particles containing silicon. The conductive additive includes carbon nanotubes and carbon black. The carbon nanotubes have an average fiber diameter of 0.5 nm or more and 6 nm or less, and an average fiber length of 1.2 μm or more and 8 μm or less. In the negative electrode mixture layer 11, the mass ratio of carbon black to carbon nanotubes (hereinafter referred to as the "CB / CNT ratio") is 10 or more and less than 40.

[0015] In this specification, active material particles that contain a carbon material as a main component and also contain silicon are classified as second active material particles. That is, in this specification, active material particles that contain silicon are second active material particles, and active material particles that do not contain silicon and have a carbon material as a main component are first active material particles.

[0016] The negative electrode mixture layer 11 in the negative electrode 10 according to the first embodiment contains, as a conductive additive, carbon nanotubes having the above-described average fiber diameter and average fiber length, and carbon black, such that the CB / CNT ratio is 10 or more and less than 40. Thus, the negative electrode 10 according to the first embodiment contains carbon black in an amount 10 times or more and less than 40 times that of the carbon nanotubes, thereby enabling the amount of carbon nanotubes to be reduced while maintaining good battery characteristics. The negative electrode mixture layer 11 contains first active material particles containing a carbon material as a main component, and second active material particles containing silicon that undergo large volume changes during charge and discharge. By satisfying the CB / CNT ratio within the above range, the negative electrode 10 according to the first embodiment can suppress poor current collection of the negative electrode active material due to volume changes of the negative electrode active material during charge and discharge, thereby achieving a good capacity retention rate, even when the amount of carbon nanotubes is reduced. Furthermore, by ensuring that the CB / CNT ratio satisfies the above range, even when the slurry for forming the negative electrode mixture layer is prepared by a general method, the increase in viscosity of the slurry can be kept relatively small. Unless otherwise specified, the slurry referred to below is a slurry for forming a negative electrode, specifically a slurry for forming a negative electrode mixture layer.

[0017] As described above, the negative electrode 10 according to embodiment 1 can reduce the amount of carbon nanotubes while maintaining good battery characteristics, and can also keep the increase in viscosity of the slurry prepared by a conventional method relatively small.

[0018] In order to achieve better battery characteristics, the CB / CNT ratio may be 15 or more, or may be 20 or more. On the other hand, in order to further suppress an increase in the viscosity of the slurry, the CB / CNT ratio may be 35 or less, or may be 30 or less.

[0019] In order to achieve better battery characteristics and further suppress an increase in the viscosity of the slurry, the CB / CNT ratio may be, for example, 15 or more and 35 or less, or 15 or more and 30 or less.

[0020] Each component of the negative electrode 10 according to the first embodiment will be specifically described below.

[0021] <Negative electrode mixture layer> [Conductive additive] (Carbon nanotubes) Carbon nanotubes are carbon fibers with a nano-sized fiber diameter and an extremely large aspect ratio (ratio of fiber length to outer diameter). Carbon fibers with a large aspect ratio form linear contacts rather than point contacts between active materials and between the active material and the current collector. Carbon fibers with excellent conductivity are interposed between active material particles, forming linear contacts with the particles. As a result, the carbon nanotubes with excellent conductivity form linear conductive paths between the active materials and between the active material and the current collector, and also form linear contacts with the current collector, improving current collection.

[0022] As described above, the carbon nanotubes contained as a conductive additive in the negative electrode mixture layer 11 have an average fiber length of 1.2 μm or more and 8 μm or less. The average fiber length of the carbon nanotubes is 1.2 μm or more. In this case, even when the volume of the negative electrode active material changes significantly due to charge and discharge, linear contact with the carbon nanotube fibers is maintained in response to the volume change, thereby maintaining electrical connection with the negative electrode active material. When the average fiber length of the carbon nanotubes is 1.2 μm or more, current collection defects are significantly suppressed. The average fiber length may be 1.5 μm or more, or may be 2 μm or more. On the other hand, as the average fiber length of the carbon nanotubes increases, the carbon nanotubes tend to aggregate, which can easily cause poor dispersion due to aggregation of the carbon nanotubes during slurry preparation and an increase in the viscosity of the slurry. Furthermore, the longer the average fiber length, the greater the carbon nanotube content in the negative electrode mixture layer 11 when securing the required number of carbon nanotubes, making it difficult to achieve high capacity. In order to achieve high capacity, and to easily prepare a slurry in which carbon nanotubes are dispersed together with a negative electrode active material and to suppress an increase in the viscosity of the slurry during the production of the negative electrode material, the average fiber length of the carbon nanotubes is set to 8 μm or less. The average fiber length may be 6 μm or less, or may be 4 μm or less. When the average fiber length of the carbon nanotubes is 4 μm or less, aggregation of the carbon nanotubes is suppressed, and it is easy to obtain a slurry in a well-dispersed state.

[0023] The average fiber length of the carbon nanotubes may be 1.2 μm or more and 6 μm or less, 1.2 μm or more and 4 μm or less, 1.5 μm or more and 4 μm or less, or 2 μm or more and 4 μm or less.

[0024] As described above, the carbon nanotubes contained as a conductive additive in the negative electrode mixture layer 11 have an average fiber diameter of 0.5 nm or more and 6 nm or less. The average fiber diameter of the carbon nanotubes may be 0.5 nm or more, preferably 1 nm or more, because they can be easily produced. On the other hand, as the average fiber diameter of the carbon nanotubes increases, the number of carbon nanotubes contained in the negative electrode mixture layer 11 decreases for a given carbon nanotube content, making it more difficult to suppress current collection defects. While increasing the carbon nanotube content in the slurry increases the tendency for carbon nanotubes to aggregate in the slurry, making uniform dispersion more difficult and increasing the viscosity of the slurry. Therefore, when producing the negative electrode mixture layer 11, the average fiber diameter of the carbon nanotubes is set to 6 nm or less in order to facilitate the preparation of a slurry in which carbon nanotubes are dispersed together with the negative electrode active material and to suppress an increase in the viscosity of the slurry. By setting the average fiber diameter to 6 nm or less, current collection defects are significantly suppressed and an easily manufactured negative electrode mixture layer 11 can be obtained. The average fiber diameter of the carbon nanotubes is preferably 4 nm or less, and more preferably 3 nm or less.

[0025] The average fiber diameter of the carbon nanotubes may be 1 nm or more and 6 nm or less, 1 nm or more and 4 nm or less, or 1 nm or more and 3 nm or less.

[0026] Here, the average fiber length of carbon nanotubes can be determined by image analysis using a scanning electron microscope (SEM). The average fiber length can be determined by, for example, measuring the fiber lengths of 100 randomly selected carbon nanotubes and averaging them. The fiber length refers to the length when the nanotubes are straight. The fiber diameter refers to the length perpendicular to the fiber length direction, and refers to the outer diameter of the carbon nanotube. It is known that carbon nanotubes may not be single fibers but may form a bundle of multiple nanotubes, called a bundle state. The average length of each carbon nanotube in this bundle state may be used as the fiber length of each carbon nanotube. The average fiber diameter of carbon nanotubes can be determined by image analysis using a transmission electron microscope (TEM). Furthermore, in the case of carbon nanotubes with two or fewer walls on the tube surface, the Raman shift in the Raman spectrum is 150 to 300 cm. -1 The diameter can be estimated by analyzing the RBM (Radial Breathing Mode) that appears in the range of 100. The average fiber diameter can be calculated by measuring the fiber diameters of, for example, 100 randomly selected carbon nanotubes and averaging them.

[0027] The carbon nanotubes preferably contain single-walled carbon nanotubes. This allows current collection defects to be effectively suppressed with a small amount of carbon nanotubes, resulting in better battery characteristics. Carbon nanotubes with an average fiber diameter of 5 nm or less contain a large amount of single-walled carbon nanotubes. The single-walled carbon nanotubes may account for 50% by mass or more, 80% by mass or more, 90% by mass or more, or even 100% by mass of the total carbon nanotubes.

[0028] The carbon nanotubes may include double-walled carbon nanotubes or multi-walled carbon nanotubes, as long as the average fiber diameter is 6 nm or less.

[0029] In the negative electrode mixture layer 11, the content of carbon nanotubes may be 0.01% by mass or more and less than 0.10% by mass relative to the entire negative electrode active material. In the negative electrode according to embodiment 1, the carbon nanotubes and carbon black are contained in the negative electrode mixture layer 11 so that the CB / CNT ratio is 10 or more and less than 40. This makes it possible to suppress current collection defects and achieve better battery characteristics even when the content of carbon nanotubes relative to the entire negative electrode active material is as low as 0.01% by mass or more and less than 0.10% by mass. Furthermore, because the content of carbon nanotubes relative to the entire negative electrode active material is less than 0.10% by mass, an increase in the viscosity of the slurry can be further suppressed.

[0030] In the negative electrode mixture layer 11, the carbon nanotube content may be 0.01% by mass or more and 0.06% by mass or less relative to the total negative electrode active material. Even if the carbon nanotube content is as low as 0.06% by mass or less relative to the total negative electrode active material, the negative electrode according to embodiment 1 can suppress current collection defects and achieve better battery characteristics. Furthermore, by further reducing the carbon nanotube content to less than 0.06% by mass relative to the total negative electrode active material, an increase in the viscosity of the slurry can be further suppressed.

[0031] The carbon nanotube content relative to the total negative electrode active material is determined from a sample obtained by extracting only the negative electrode mixture layer from a discharged secondary battery. Specifically, the discharged secondary battery is first disassembled to extract the negative electrode. Next, the negative electrode is washed with an organic solvent and further vacuum-dried, and then the negative electrode mixture layer is peeled off to obtain a sample. The pulverized sample is dispersed in a dispersion medium such as water and / or alcohol and centrifuged to separate the carbon nanotubes. Furthermore, thermal analysis such as TG-DTA (Thermogravimetry-Differential Thermal Analysis) can be performed on the sample to calculate the ratio of binder components and conductive material components other than the negative electrode active material. Microscopic Raman spectroscopy can be performed on a cross-section of the negative electrode mixture layer 11 to identify carbon species such as carbon nanotubes and acetylene black, and their ratios can be calculated from thermal analysis such as TG-DTA of the peeled sample.

[0032] (Carbon Black) Carbon black is contained as a conductive additive in the negative electrode mixture layer 11 so as to satisfy the above-mentioned CB / CNT ratio. Any known carbon black can be used as the carbon black, and there are no particular limitations on the carbon black.

[0033] The content of carbon black in the negative electrode mixture layer 11 may be 0.5% by mass or more and 2.5% by mass or less relative to the negative electrode active material, thereby suppressing current collection defects and realizing better battery characteristics, while further suppressing an increase in the viscosity of the slurry.

[0034] The content of carbon black in the negative electrode mixture layer 11 may be 0.5% by mass or more and 1.5% by mass or less relative to the negative electrode active material, thereby suppressing current collection defects and realizing better battery characteristics, while further suppressing an increase in the viscosity of the slurry.

[0035] The carbon black content relative to the negative electrode active material can be determined by the following method. Only the negative electrode mixture layer 11 is peeled off from the negative electrode 10, and the resulting powder is pulverized and used for TG-DTA analysis in a nitrogen atmosphere. Based on the results of a previous measurement performed using only carbon black, the carbon black content is determined from the weight loss at 600°C or higher.

[0036] [Negative Electrode Active Material] (First Active Material Particles) The first active material particles are active material particles containing a carbon material as a main component. Here, the main component means the component that is contained in the largest amount by mass.

[0037] The carbon material may be graphite. That is, the first active material particles may be mainly composed of graphite. Among carbon materials, graphite is preferred because it has excellent charge / discharge stability and a small irreversible capacity. Note that graphite refers to a material with a developed graphite-type crystal structure, and generally refers to a carbon material in which the average interplanar spacing d002 of the (002) plane measured by X-ray diffraction is 0.340 nm or less.

[0038] Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. The graphite may be any known graphite used as a negative electrode active material. A carbon material other than graphite may also be included in the negative electrode active material.

[0039] The first active material particles may contain a carbon material other than graphite as a main component, such as easily graphitizable carbon (soft carbon) or hardly graphitizable carbon (hard carbon).

[0040] The carbon material may be used alone or in combination of two or more.

[0041] The first active material particles may have an average particle size of, for example, 5 μm or more and 30 μm or less. The first active material particles having such an average particle size are effective in suppressing poor current collection caused by the conductive additive containing carbon nanotubes and carbon black at the above-mentioned CB / CNT ratio.

[0042] The average particle size of the first active material particles is determined by observing the cross section of the negative electrode mixture layer using a SEM or TEM. The particle type is identified by micro-Raman spectroscopy, and the grain boundaries of each particle are determined from a cross-sectional photograph. The particle size is determined as the diameter of a circle equal to the area defined by the grain boundaries. At least 100 particles are randomly selected, and the average particle size is determined.

[0043] The median diameter (D50) at which the cumulative volume reaches 50% in the volume-based particle size distribution can be used to determine the average particle diameter of the first active material particles before forming the negative electrode mixture layer 11. The median diameter can be determined using, for example, a laser diffraction / scattering particle size distribution analyzer.

[0044] The content of the first active material particles in the negative electrode active material may be, for example, 40% by mass or more and 90% by mass or less, which can increase the capacity of the battery.

[0045] (Second Active Material Particles) The second active material particles are particles containing silicon (hereinafter referred to as "silicon-containing particles").

[0046] As used herein, the term "silicon-containing particles" refers to particles of a material containing Si. Examples of materials containing Si include simple Si, Si alloys, Si compounds (such as Si oxides), and composite materials containing Si.

[0047] Examples of silicon oxide include SiO x X may be, for example, 0.5≦X<2, 0.5≦X<1.6, or 0.8≦X≦1.6.

[0048] The silicon-containing particles may be, for example, particles of a composite material containing Si (hereinafter referred to as "composite particles").

[0049] For example, the second active material particles (i.e., silicon-containing particles) may comprise composite particles including a matrix containing at least one element selected from the group consisting of carbon and oxygen and a silicon phase dispersed in the matrix. The matrix functions, for example, as an ion-conducting phase. Using such composite particles as a negative electrode active material can increase the capacity of a battery. On the other hand, silicon-containing particles undergo large volume changes due to the absorption and desorption of lithium ions, making them prone to current collection failures due to repeated charge and discharge. When current collection failure occurs, electrical connection between the silicon-containing particles and other negative electrode active materials or the current collector is severed, resulting in isolation of the silicon-containing particles. Isolated particles cannot contribute to capacity, resulting in current collection failure and a reduced capacity retention rate. However, because the negative electrode mixture layer 11 contains a conductive additive containing carbon nanotubes and carbon black at the above-mentioned CB / CNT ratio, current collection failure is effectively suppressed, enabling a high capacity retention rate to be maintained.

[0050] As the matrix containing at least one element selected from the group consisting of carbon and oxygen, i.e., the ion-conducting phase, for example, at least one selected from the group consisting of a SiO2 phase, a silicate phase, and a carbon phase can be used.

[0051] Examples of silicon-containing particles include SiO XAt least one type of particle selected from the group consisting of first particles containing silicon oxide represented by the formula (0.5≦X<1.6), second particles containing a lithium silicate phase and a silicon phase dispersed in the lithium silicate phase, and third particles containing a carbon phase and a silicon phase dispersed in the carbon phase may be used. Using silicon-containing particles as the second active material particles can increase the battery capacity. However, silicon-containing particles undergo large volume changes due to the absorption and desorption of lithium ions, making them prone to current collection failures with repeated charge and discharge. When current collection failure occurs, electrical connection between the silicon-containing particles and other negative electrode active materials or the current collector is severed, resulting in isolation of the silicon-containing particles. Isolated particles cannot contribute to capacity, resulting in current collection failure and a reduced capacity retention rate. However, when the negative electrode mixture layer 11 contains a conductive additive containing carbon nanotubes and carbon black at the above-mentioned CB / CNT ratio, current collection failure is effectively suppressed, enabling a high capacity retention rate to be maintained.

[0052] The silicon-containing particles may include multiple types of particles selected from the group consisting of first particles, second particles, and third particles. For example, the silicon-containing particles may be composed of two types of particles selected from these, or may include all three types of particles. Specifically, the silicon-containing particles may include first particles and second particles, first particles and third particles, or second particles and third particles. Alternatively, the silicon-containing particles may include all of the first, second, and third particles.

[0053] (First Particles) The first particles are a mixture of SiO2 and Si fine particles at a microscopic level. X The first particles may include silicon oxide particles and a carbon layer disposed around the silicon oxide particles.

[0054] (Second Particles) The second particles contain a lithium silicate phase and silicon particles (silicon phase) dispersed in the lithium silicate phase. The lithium silicate phase contains Li 2Z SiO(2+Z) The lithium silicate phase may contain or be composed of a lithium silicate represented by the formula (0<Z<2). Z preferably satisfies the relationship 0<Z<1. 50% by mass or more (e.g., 60% by mass or more) of the lithium silicate phase may be composed of a lithium silicate that satisfies 0<Z≦0.5.

[0055] The second particles may include at least one element Me dispersed within the lithium silicate phase. The at least one element Me is at least one element selected from the group consisting of rare earth elements and alkaline earth metal elements. Examples of alkaline earth metal elements include Mg, Ca, Sr, Ba, etc.

[0056] The element Me may be dispersed in the lithium silicate phase as Me oxide. The Me oxide may include at least one selected from the group consisting of yttrium oxide, cerium oxide, calcium oxide, and magnesium oxide. The lithium silicate phase may include zirconium oxide. The element Me may be dispersed in the zirconium oxide.

[0057] The amount of element Me contained in the second particles can be calculated assuming that element Me forms a stoichiometric oxide (estimated amount of Me oxide), regardless of the state of element Me or the type of compound of element Me. The estimated amount of Me oxide may be in the range of 0.001% by mass or more and 1.0% by mass or less with respect to the total amount of the lithium silicate phase and the silicon phase. By setting the estimated amount of Me oxide to 0.001% by mass or more, the effect of reducing the reaction area and improving the hardness of the lithium silicate phase is enhanced. On the other hand, by setting the estimated amount of Me oxide to 1.0% by mass or less, the decrease in initial capacity can be suppressed.

[0058] The lithium silicate phase may contain a metal compound such as a metal oxide, a metal carbide, a metal nitride, or a metal boride. Suitable metal compounds are metal oxides and metal carbides. Among these, it is preferable to use at least one selected from the group consisting of zirconium oxide (ZrO), aluminum oxide (AlO), zirconium carbide (ZrC), tungsten carbide (WC), and silicon carbide (SiC). The amount of the compound of a metal element other than element Me may be in the range of 0.005% by mass to 15% by mass, 0.01% by mass to 10% by mass, or 0.01% by mass to 1% by mass, based on the total amount of the lithium silicate phase and the silicon phase. The amount of the compound of the metal element, similar to the content of element Me, may be calculated by assuming that the metal element forms a stoichiometric oxide.

[0059] The crystallite size of the silicon phase dispersed in the lithium silicate phase is, for example, 10 nm or more. The silicon phase is a particulate phase of simple silicon (Si). When the crystallite size of the silicon phase is 10 nm or more, the surface area of ​​the silicon phase can be kept small, making it less likely for the silicon phase to deteriorate, which would otherwise lead to the generation of irreversible capacity. The crystallite size of the silicon phase is calculated using the Scherrer equation from the half-width of the diffraction peak assigned to the Si(111) plane in the X-ray diffraction (XRD) pattern of the silicon phase.

[0060] The average particle size of the silicon phase in the second particles before the first charge is, for example, 500 nm or less, or may be 200 nm or less, or may be 50 nm or less. After the first charge, the average particle size of the silicon phase is, for example, 400 nm or less, or may be 100 nm or less. By miniaturizing the silicon phase, the volume change during charge and discharge is reduced, and the structural stability of the second particles is further improved. The average particle size of the silicon phase in the second particles can be measured using SEM or TEM images obtained by SEM or TEM observation of the second particles in which the cross section of the silicon phase is exposed. Specifically, the average particle size of the silicon phase is determined by averaging the maximum diameters of 100 silicon phases arbitrarily selected from the cross-sectional SEM or TEM image of the second particles.

[0061] The content of the silicon phase (elementary Si) in the second particles is, for example, in the range of 20% by mass or more and 95% by mass or less, or may be in the range of 35% by mass or more and 75% by mass or less, from the viewpoint of increasing capacity and improving cycle characteristics. This range improves lithium ion diffusibility and facilitates obtaining excellent load characteristics. Furthermore, the surface of the silicon phase that is exposed without being covered by the lithium silicate phase is reduced, suppressing side reactions between the non-aqueous electrolyte and the silicon phase.

[0062] The second particles may include a conductive material that coats at least a portion of their surfaces. Because the lithium silicate phase has poor electronic conductivity, the conductivity of the second particles also tends to be low. Coating the surfaces with a conductive material can dramatically increase the conductivity. The conductive layer is preferably thin enough that it does not substantially affect the average particle size of the second particles. For example, from the viewpoint of ensuring conductivity and lithium ion diffusibility, the thickness of the conductive layer may be 1 nm or more and 200 nm or less, or 5 nm or more and 100 nm or less. Examples of materials and methods for forming the conductive layer will be described later.

[0063] (Third Particles) The third particles include a carbon phase and a silicon phase dispersed in the carbon phase. The carbon phase of the third particles may be composed of amorphous carbon (i.e., amorphous carbon). The amorphous carbon may be hard carbon, soft carbon, or other. Amorphous carbon generally refers to a carbon material in which the average interplanar spacing d002 of the (002) plane measured by X-ray diffraction exceeds 0.34 nm. The carbon phase of the third particles is conductive. Therefore, even if voids are formed around the third particles, contact between the third particles and their surroundings is easily maintained. As a result, capacity loss due to repeated charge / discharge cycles is easily suppressed.

[0064] The content of the silicon phase in the third particles may be 30% by mass or more and 80% by mass or less, or 40% by mass or more and 70% by mass or less, in which case the capacity of the negative electrode is sufficiently high and the cycle characteristics are likely to be improved.

[0065] The average particle size of the silicon phase in the third particles may be, for example, 1 nm or more. The average particle size of the silicon phase may be 1000 nm or less, 500 nm or less, 200 nm or less, 100 nm or less, or 50 nm or less. The finer the silicon phase, the smaller the volume change of the third particles during charging and discharging, improving the structural stability of the third particles. The average particle size of the silicon phase in the third particles can be determined by the same method as for the average particle size of the silicon phase in the second particles described above.

[0066] The composition and component contents of the second and third particles can be analyzed by the method described in WO 2018 / 179969.

[0067] The content of each element contained in the silicon-containing particles may be measured, for example, by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Specifically, the silicon-containing particles are dissolved in a heated acid solution, and the carbon remaining in the solution is removed by filtration. The resulting filtrate is then analyzed by ICP-AES to measure the spectral intensity of each element. Subsequently, a calibration curve is created using commercially available standard solutions of each element, and the content of each element is calculated.

[0068] The second particles and the third particles each have a so-called sea-island structure. The silicon phase (islands) in the second and third particles are dispersed in a matrix (sea) of silicate phase and carbon phase, respectively, and are covered with an ion-conducting phase (silicate phase and carbon phase). The sea-island structure limits contact between the silicon phase and the electrolyte, thereby suppressing side reactions. In addition, stress generated by the expansion and contraction of the silicon phase is alleviated by the matrix of the ion-conducting phase.

[0069] The average particle size of the second active material particles may be, for example, 3 μm or more and 15 μm or less. Second active material particles having such an average particle size have a significant effect of suppressing poor current collection caused by the conductive additive containing carbon nanotubes and carbon black at the above-mentioned CB / CNT ratio. The average particle size of the second active material particles can be determined by the same method as that for the average particle size of the first active material particles.

[0070] The content of the second active material particles in the negative electrode active material may be, for example, 10% by mass or more and 60% by mass or less, which can increase the capacity of the battery.

[0071] The content of the second active material particles in the negative electrode active material may be, for example, 30% by mass or more and 60% by mass or less, which can further increase the capacity of the battery.

[0072] (Method for producing first particles) SiO X can be produced by, for example, a vapor deposition method. X The particles may be coated with carbon. X The particles are crushed and classified to adjust the particle size. Next, the surfaces of the obtained particles are coated with carbon by a CVD method under an argon atmosphere. Then, the particles are crushed and classified to obtain SiO X The first particles are prepared as follows: X The method for coating particles with carbon is not limited to the above method, and various well-known methods can be used. X The process of coating the particles with carbon may be omitted.

[0073] (Method for producing second particles) Next, an example of a method for producing second particles will be described in detail. The second particles may be produced by a method other than the production method described below. The second particles may be produced by the method described in WO 2018 / 179969.

[0074] The second particles are generally synthesized through two processes: a front-end process of obtaining lithium silicate, and a back-end process of obtaining second particles from lithium silicate and raw silicon. When the element Me is added, the element Me may be added to the lithium silicate raw material in the front-end process, but it is preferable to add it in the back-end process so as not to affect the synthesis of lithium silicate. More specifically, the method for producing the second particles preferably includes a step (i) of mixing silicon dioxide and a lithium compound and firing the resulting mixture to obtain lithium silicate, and a step (ii) of compounding the lithium silicate with raw silicon (and optionally the element Me) to obtain second particles containing a lithium silicate phase and a silicon phase dispersed within the lithium silicate phase.

[0075] (Step (i)) Formula: Li 2Z SiO 2+Z The value of Z of the lithium silicate expressed by the formula (1) can be controlled by the atomic ratio of silicon to lithium, Li / Si, in the mixture of silicon dioxide and a lithium compound. To synthesize a high-quality lithium silicate with little elution of alkaline components, it is preferable to make Li / Si smaller than 1.

[0076] The lithium compound may be lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. These may be used alone or in combination of two or more.

[0077] The mixture containing silicon dioxide and the lithium compound is preferably heated in air at 400°C or higher and 1200°C or lower, preferably 800°C or higher and 1100°C or lower, to react the silicon dioxide with the lithium compound.

[0078] (Step (ii)) Next, lithium silicate and raw material silicon are composited. For example, a mixture of lithium silicate and raw material silicon (which may further contain element Me) may be pulverized while applying shear force to the mixture. Coarse silicon particles having an average particle size of several μm or more and several tens of μm or less may be used as the raw material silicon. It is preferable to control the crystallite size of the silicon phase finally obtained to 10 nm or more, calculated by Scherrer's formula from the half-width of the diffraction peak assigned to the Si (111) plane in the XRD pattern.

[0079] The material for the element Me used in the charging may be an oxide, oxalate, nitrate, sulfate, halide, carbonate, or the like of the element Me. Among these, Me oxide is preferred because it is stable and has good ionic conductivity. More specifically, CeO2, Sc2O3, YO3, Er2O3, Tm2O3, Yb2O3, Lu2O3, and the like may be used. Compounds containing elements other than the element Me and oxygen, such as yttria-stabilized zirconia, may also be used. These may be used alone or in combination of two or more.

[0080] For example, lithium silicate and raw silicon (and optionally a compound of element Me) may be mixed in a predetermined mass ratio, and the mixture may be stirred while being pulverized using a grinding device such as a ball mill. However, the composite process is not limited to this. For example, silicon nanoparticles and lithium silicate nanoparticles (and optionally a compound of element Me) may be synthesized and then mixed without using a grinding device.

[0081] Next, the microparticulated mixture is heated and fired, for example, in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere) at 450°C or higher and 1000°C or lower. At this time, the mixture may be fired while applying pressure using a hot press or the like to produce a sintered body of the mixture. Lithium silicate is stable at temperatures between 450°C and 1000°C, for example, and hardly reacts with silicon, so any capacity loss is minor. During firing, the silicate softens and flows to fill the gaps between the silicon phases. As a result, a dense block-shaped sintered body can be obtained, with the silicate phase forming the sea portion and the silicon phase forming the island portion.

[0082] The sintered body may then be pulverized into granules to obtain second particles. By appropriately selecting the pulverization conditions, second particles having an average particle size within the above-mentioned range can be obtained.

[0083] After step (ii), step (iii) may be carried out in which at least a portion of the surface of the second particles is coated with a conductive material to form a conductive layer. The conductive material is preferably electrochemically stable, and a carbon material is preferred. A CVD method using a hydrocarbon gas such as acetylene or methane as a raw material may be used as a method for coating the surface of the particulate material with a carbon material. Alternatively, a method may be used in which coal pitch, petroleum pitch, phenolic resin, or the like is mixed with the second particles and then heated. Carbon black may also be attached to the surface of the second particles.

[0084] A step of washing the second particles with an acid may be performed. For example, the second particles may be washed with an acidic aqueous solution. By washing with an acid, trace amounts of components such as Li2SiO3 that may be generated when the raw silicon and lithium silicate are combined can be dissolved and removed. As the acidic aqueous solution, an aqueous solution of an inorganic acid such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, or carbonic acid, or an aqueous solution of an organic acid such as citric acid or acetic acid can be used.

[0085] (Method for Producing Third Particles) As examples of the method for producing the third particles, first and second methods will be described below. The third particles may be produced by a method other than the production methods described below.

[0086] In the first method, raw silicon and a carbon source are first mixed, and then the mixture of raw silicon and carbon source is pulverized and composited using a pulverizing device such as a ball mill while being finely divided. An organic solvent may be added to the mixture for wet pulverization. At this time, the raw silicon is pulverized to form a silicon phase. The silicon phase is dispersed in the carbon source matrix.

[0087] Examples of carbon sources that can be used include, but are not limited to, water-soluble resins such as carboxymethyl cellulose (CMC), hydroxyethyl cellulose, polyacrylates, polyacrylamide, polyvinyl alcohol, polyethylene oxide, and polyvinylpyrrolidone; sugars such as cellulose and sucrose; petroleum pitch, coal pitch, and tar.

[0088] As the organic solvent, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. can be used.

[0089] Next, the composite of the silicon phase and the carbon source is heated in an inert gas atmosphere (e.g., an argon, nitrogen, or the like) to, for example, 700°C or higher and 1200°C or lower. This heating carbonizes the carbon source to produce amorphous carbon. This results in third particles in which the silicon phase is dispersed in the carbon phase containing amorphous carbon.

[0090] In the second method, raw silicon and a carbon material are first mixed, and then the mixture of raw silicon and carbon material is pulverized and composited using a pulverizing device such as a ball mill while being finely divided. An organic solvent may be added to the mixture for wet pulverization. At this time, the raw silicon is pulverized to form a silicon phase. The silicon phase is dispersed in the matrix of the carbon material.

[0091] The composite of the raw silicon and the carbon material as described above provides third particles in which the silicon phase is dispersed in the carbon phase of amorphous carbon. The third particles may then be heated to, for example, 700°C or higher and 1200°C or lower in an inert gas atmosphere.

[0092] [Binder] The negative electrode mixture layer 11 may further contain, for example, a binder. The binder may include at least one fluorine-free polymer material selected from the group consisting of carboxymethyl cellulose (CMC) and its alkali metal salts, polyacrylic acid and its alkali metal salts, and copolymers containing butadiene units. Fluorine-containing polymers, including polytetrafluoroethylene (PTFE), are relatively expensive compared to common fluorine-free materials. Furthermore, fluorine-containing polymers have issues with reduction resistance. When used in the negative electrode of a lithium-ion battery, fluorine-containing polymers are electrochemically reduced and degraded by decomposition reactions such as defluorination due to the low reaction potential of the active material. Using a fluorine-free polymer material not only reduces costs but also suppresses the deterioration of electrode performance due to electrochemical reduction degradation.

[0093] <Negative electrode current collector> The negative electrode current collector 12 may be a sheet or film made of a metal material such as stainless steel, nickel, copper, or an alloy thereof. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material such as carbon may be applied to the surface of the negative electrode current collector 12 as a conductive auxiliary material.

[0094] The thickness of the negative electrode current collector 12 is not particularly limited, but may be, for example, 1 μm or more and 50 μm or less, or 5 μm or more and 20 μm or less, from the viewpoint of balancing the strength and weight reduction of the negative electrode 10.

[0095] <Method for Manufacturing Negative Electrode> For example, a slurry is prepared by dispersing a negative electrode mixture containing first active material particles, second active material particles, carbon nanotubes having an average fiber diameter of 0.5 nm or more and 6 nm or less and an average fiber length of 1.2 μm or more and 8 μm or less, and carbon black in a dispersion medium. This slurry is applied to the surface of the negative electrode current collector 12 to form a coating film, and the coating film is then dried to form the negative electrode mixture layer 11. The composition of the slurry may be determined depending on the components of the negative electrode 10 to be manufactured, and for example, a binder may be further added.

[0096] In the prepared slurry, the carbon nanotubes and carbon black are added to the slurry so that the CB / CNT ratio satisfies 10 or more and less than 40.

[0097] (Embodiment 2) A battery according to Embodiment 2 includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode according to Embodiment 1. With this configuration, the battery according to Embodiment 2 allows the amount of carbon nanotubes in the negative electrode containing carbon nanotubes to be reduced while maintaining good battery characteristics (specifically, capacity retention rate), and also makes it possible to keep the increase in viscosity of a slurry for forming the negative electrode prepared by a general method relatively small.

[0098] 2 is a longitudinal cross-sectional view schematically illustrating an example of a battery according to embodiment 2. The battery 100 is a cylindrical battery including a cylindrical battery case, a wound electrode group 24, and an electrolyte (not shown). The electrode group 24 is housed in the battery case and is in contact with the electrolyte.

[0099] The battery case is composed of a case body 25, which is a cylindrical metal container with a bottom, and a sealing body 26 that seals the opening of the case body 25. A gasket 37 is disposed between the case body 25 and the sealing body 26. The gasket 37 ensures the airtightness of the battery case. Within the case body 25, insulating plates 27 and 28 are disposed on both ends of the electrode group 24 in the direction of the winding axis of the electrode group 24, respectively.

[0100] Case body 25 has, for example, a step portion 31. Step portion 31 can be formed by partially pressing the side wall of case body 25 from the outside. Step portion 31 may be formed in an annular shape on the side wall of case body 25 along the circumferential direction of an imaginary circle defined by case body 25. In this case, sealing body 26 is supported by, for example, the surface of step portion 31 on the opening side.

[0101] Sealing body 26 includes a filter 32, a lower valve body 33, an insulating member 34, an upper valve body 35, and a cap 36. These components are stacked in this order in sealing body 26. Sealing body 26 is attached to the opening of case body 25 so that cap 36 is located on the outside of case body 25 and filter 32 is located on the inside of case body 25.

[0102] Each of the above-mentioned members constituting the sealing body 26 has, for example, a disk or ring shape. The above-mentioned members, except for the insulating member 34, are electrically connected to one another.

[0103] The electrode group 24 has a positive electrode 21, a separator 22, and a negative electrode 23. The positive electrode 21, the separator 22, and the negative electrode 23 are all strip-shaped. The width direction of the strip-shaped positive electrode 21 and the negative electrode 23 is, for example, parallel to the winding axis of the electrode group 24. The separator 22 is disposed between the positive electrode 21 and the negative electrode 23. The positive electrode 21 and the negative electrode 23 are spirally wound with the separator 22 interposed between these electrodes.

[0104] When observing the cross section of the battery 100 in a direction perpendicular to the winding axis of the electrode group 24, the positive electrodes 21 and negative electrodes 23 are stacked alternately in the radial direction of an imaginary circle defined by the case body 25, with a separator 22 interposed between these electrodes.

[0105] The positive electrode 21 is electrically connected to a cap 36, which also serves as a positive electrode terminal, via a positive electrode lead 29. One end of the positive electrode lead 29 is connected, for example, to near the center of the positive electrode 21 in the longitudinal direction of the positive electrode 21. The positive electrode lead 29 passes through a through-hole formed in the insulating plate 27 and extends from the positive electrode 21 to the filter 32. The other end of the positive electrode lead 29 is welded, for example, to the surface of the filter 32 on the electrode group 24 side.

[0106] The negative electrode 23 is electrically connected to the case body 25, which also serves as a negative electrode terminal, via a negative electrode lead 30. One end of the negative electrode lead 30 is connected to, for example, an end of the negative electrode 23 in the longitudinal direction of the negative electrode 23. The other end of the negative electrode lead 30 is welded to, for example, the inner bottom surface of the case body 25.

[0107] Each component of the battery 100 will be specifically described below.

[0108] The positive electrode 21 includes a material having the property of absorbing and releasing metal ions (e.g., lithium ions). The positive electrode 21 includes, for example, a positive electrode active material. The positive electrode 21 includes, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector.

[0109] The positive electrode current collector can be, for example, a sheet or film made of a metal material such as aluminum, stainless steel, titanium, or an alloy thereof. Aluminum and its alloys are suitable as materials for the positive electrode current collector because they are inexpensive and easy to form into thin films. The sheet or film may be porous or non-porous. Metal foil, metal mesh, or the like may be used as the sheet or film. A carbon material such as carbon may be applied to the surface of the positive electrode current collector as a conductive auxiliary material.

[0110] The positive electrode mixture layer includes a positive electrode active material. The positive electrode active material can be a material capable of absorbing and releasing metal ions (e.g., lithium ions). Examples of the positive electrode active material include lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, using a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate as the positive electrode active material can reduce battery manufacturing costs and increase average discharge voltage. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate.

[0111] The positive electrode mixture layer may further contain a binder. As the binder, the materials described in the first embodiment as binders usable in the negative electrode mixture layer can also be used in the positive electrode mixture layer.

[0112] The positive electrode mixture layer may further contain a conductive additive. As the conductive additive, the materials described in the first embodiment as conductive additives that can be used in the negative electrode mixture layer can also be used in the positive electrode mixture layer.

[0113] The negative electrode 23 includes a material having the property of absorbing and releasing metal ions (for example, lithium ions). The negative electrode 23 is the negative electrode 10 according to the first embodiment.

[0114] The electrolyte solution used as the electrolyte may contain a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolyte solution may be, for example, 0.5 mol / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte solution having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0115] The non-aqueous solvent may be a cyclic carbonate, a chain carbonate, a cyclic ether, a chain ether, a nitrile, an amide, etc. One selected from these solvents may be used, or two or more may be used in combination.

[0116] Examples of lithium salts that can be used include lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. One selected from these electrolyte salts may be used, or two or more may be used in combination.

[0117] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator 22 has high ion permeability and adequate mechanical strength and insulating properties. The separator 22 can be made of a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The separator 22 can be made of a polymer, for example. The polymer can be a polyolefin such as polypropylene or polyethylene.

[0118] In the battery according to the second embodiment, the electrolyte may be impregnated into a polymer provided as a separator, for example, i.e., the battery according to the second embodiment may have a structure in which the electrolyte and the polymer are used in combination.

[0119] The battery according to the second embodiment may further include a solid electrolyte as the electrolyte. That is, the battery according to the present disclosure may have a hybrid structure in which an electrolytic solution and a solid electrolyte are used in combination. Examples of solid electrolyte materials include halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, and organic polymer solid electrolytes. In the present disclosure, the term "halide solid electrolyte" refers to a solid electrolyte containing a halogen element as the main component of the anions. The term "sulfide solid electrolyte" refers to a solid electrolyte containing sulfur as the main component of the anions. The term "oxide solid electrolyte" refers to a solid electrolyte containing oxygen as the main component of the anions. The term "main component of the anions" refers to the anion with the largest mass among all the anions constituting the solid electrolyte.

[0120] As an example of the structure of the battery according to the second embodiment, the configuration example shown in FIG. 2 is described, i.e., a cylindrical nonaqueous electrolyte secondary battery in which a wound electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and an electrolyte solution are housed in an outer casing. However, the battery according to the present disclosure is not limited to this configuration example. The battery according to the second embodiment may have any shape, such as a prismatic shape, a coin shape, a button shape, or a laminate shape. Furthermore, instead of the wound electrode group in the battery according to the second embodiment, an electrode group of another shape, such as an electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween, may be used.

[0121] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0122] (Technology 1) An anode comprising a negative electrode mixture layer containing a negative electrode active material and a conductive additive, wherein the negative electrode active material contains first active material particles containing a carbon material as a main component and second active material particles containing silicon, the conductive additive contains carbon nanotubes and carbon black, the carbon nanotubes have an average fiber diameter of 0.5 nm or more and 6 nm or less and an average fiber length of 1.2 μm or more and 8 μm or less, and the mass ratio of the carbon black to the carbon nanotubes is 10 or more and less than 40.

[0123] This configuration allows the amount of carbon nanotubes in a negative electrode containing carbon nanotubes to be reduced while maintaining good battery characteristics, and also makes it possible to keep the increase in viscosity of the slurry used to form a negative electrode prepared by a conventional method relatively small.

[0124] (Technology 2) The negative electrode according to Technology 1, wherein the carbon nanotubes include single-walled carbon nanotubes.

[0125] This allows current collection defects to be effectively suppressed with a small amount of carbon nanotubes, resulting in better battery characteristics.

[0126] (Technology 3) The negative electrode according to Technology 1 or 2, wherein a mass ratio of the carbon black to the carbon nanotubes is 15 or more and 30 or less.

[0127] This configuration makes it possible to achieve better battery characteristics and further suppress an increase in the viscosity of the slurry.

[0128] (Technology 4) The negative electrode according to any one of Technologies 1 to 3, wherein the content of the carbon nanotubes in the negative electrode mixture layer is 0.01% by mass or more and less than 0.10% by mass with respect to the negative electrode active material.

[0129] This configuration makes it possible to achieve better battery characteristics and further suppress an increase in the viscosity of the slurry.

[0130] (Technology 5) The negative electrode according to Technology 4, wherein the content of the carbon nanotubes in the negative electrode mixture layer is 0.01% by mass or more and 0.06% by mass or less with respect to the negative electrode active material.

[0131] This configuration makes it possible to achieve better battery characteristics and further suppress an increase in the viscosity of the slurry.

[0132] (Technology 6) The negative electrode according to any one of Technologies 1 to 5, wherein the content of the carbon black in the negative electrode mixture layer is 0.5 mass % or more and 2.5 mass % or less with respect to the negative electrode active material.

[0133] This configuration makes it possible to achieve better battery characteristics and further suppress an increase in the viscosity of the slurry.

[0134] (Technology 7) The negative electrode according to Technology 6, wherein the content of the carbon black in the negative electrode mixture layer is 0.5% by mass or more and 1.5% by mass or less with respect to the negative electrode active material.

[0135] This configuration makes it possible to achieve better battery characteristics and further suppress an increase in the viscosity of the slurry.

[0136] (Technology 8) The negative electrode according to any one of Technologies 1 to 7, wherein the second active material particles include composite particles including a matrix containing at least one element selected from the group consisting of carbon and oxygen, and a silicon phase dispersed in the matrix.

[0137] This configuration makes it possible to achieve better battery characteristics.

[0138] (Technology 9) The negative electrode according to any one of Techniques 1 to 8, wherein the second active material particles have an average particle size of 3 μm or more and 15 μm or less.

[0139] This configuration makes it possible to achieve better battery characteristics.

[0140] (Technology 10) The negative electrode according to any one of Techniques 1 to 9, wherein a content ratio of the second active material particles in the negative electrode active material is 10% by mass or more and 60% by mass or less.

[0141] This configuration makes it possible to increase the capacity of the battery.

[0142] (Technology 11) The negative electrode according to Technology 9, wherein the content of the second active material particles in the negative electrode active material is 30 mass % or more and 60 mass % or less.

[0143] This configuration makes it possible to increase the capacity of the battery.

[0144] (Technology 12) The negative electrode according to any one of Techniques 1 to 11, wherein the first active material particles contain graphite as a main component.

[0145] This configuration makes it possible to increase the capacity of the battery.

[0146] (Technology 13) The negative electrode according to any one of Techniques 1 to 12, wherein the first active material particles have an average particle size of 5 μm or more and 30 μm or less.

[0147] This configuration makes it possible to increase the capacity of the battery.

[0148] (Technology 14) The negative electrode according to any one of Technologies 1 to 13, wherein the negative electrode mixture layer further contains a binder, and the binder contains at least one fluorine-free polymer material selected from the group consisting of carboxymethyl cellulose and alkali metal salts thereof, polyacrylic acid and alkali metal salts thereof, and copolymers containing butadiene units.

[0149] This configuration makes it possible to increase the capacity of the battery.

[0150] (Technology 15) A battery comprising: the negative electrode according to any one of claims 1 to 15; a positive electrode; and an electrolyte.

[0151] This configuration allows the amount of carbon nanotubes in a negative electrode containing carbon nanotubes to be reduced while maintaining good battery characteristics, and also makes it possible to keep the increase in viscosity of the slurry used to form a negative electrode prepared by a conventional method relatively small.

[0152] The present disclosure will be described in more detail below using examples. The following examples are merely examples of embodiments, and are not intended to limit the scope of the present disclosure.

[0153] <Preparation of Negative Electrode> [Examples 1 to 6 and Comparative Examples 1 to 5] A negative electrode active material, sodium polyacrylate (PAA-Na), sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), carbon nanotubes (CNT), carbon black (CB) (Super C65), and water were mixed in a predetermined mass ratio to prepare a negative electrode mixture slurry. Single-walled carbon nanotubes were used as the carbon nanotubes. The single-walled carbon nanotubes used had an average fiber length of 5 μm and an average fiber diameter of 1.6 nm. The CB / CNT ratio in the negative electrode mixture is shown in Table 1. For the negative electrode active material, graphite was used as the first active material particles, and silicon-containing particles were used as the second active material particles.

[0154] Next, a coating film was formed by applying the negative electrode mixture slurry to the surface of a copper foil (negative electrode current collector). The coating film was dried and then rolled. In this manner, a negative electrode mixture layer was formed on both sides of the copper foil. The contents of carbon nanotubes and carbon black in the negative electrode mixture slurry were the mass % shown in Table 1, where the negative electrode active material (the total of graphite and silicon-containing particles) was taken as 100 mass %. The mixing ratio of the negative electrode active material, sodium polyacrylate, sodium carboxymethyl cellulose, and styrene-butadiene rubber in the negative electrode slurry was negative electrode active material:PAA-Na:CMC-Na:SBR=100:0.5:1:1 by mass. The mass ratio of graphite to silicon-containing particles in the negative electrode active material was graphite:silicon-containing particles=50:50.

[0155] The silicon-containing particles, which are the second active material particles, were composite particles containing Si and C, i.e., the above-mentioned third particles containing a carbon phase and a silicon phase, which were prepared by the following method. The third particles were prepared as follows.

[0156] Tetraethylorthosilane (TEOS) and cetyltrimethylammonium bromide (CTAB) were mixed in an ethanol / water / ammonia mixture to prepare CTAB-modified SiO nanoparticles. Resorcinol, formaldehyde, and a surfactant (Pluronic F-127) were added to the mixture and polymerized to obtain polymer particles encapsulating the SiO nanoparticles. The molar ratio of surfactant to resorcinol (surfactant / resorcinol) was set to 0.005, and the mass ratio of resorcinol to TEOS (resorcinol / TEOS) was approximately 0.5 / 1. The polymer particles were dried and then carbonized at 800°C in a nitrogen atmosphere. The mixture was then mixed with magnesium powder and heated at 650°C in an argon atmosphere to induce a magnesium-thermal reduction reaction. MgO was dissolved from the particles after the reaction in a mixed solution of HCl / HO / ethanol, and the particles were washed with ethanol and then dried to produce mesoporous silicon-containing particles containing Si and C and having an average particle size of 8 μm. These silicon-containing particles were designated as third particles.

[0157] <Preparation of Positive Electrode> As the positive electrode active material, LiNi 0.88 Co 0.09 Al 0.03 O2 was used. A positive electrode active material, acetylene black, polyvinylidene fluoride, and N-methyl-2-pyrrolidone (NMP) were mixed in a predetermined mass ratio to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of an aluminum foil serving as a positive electrode current collector, and the coating was dried and then rolled to form a positive electrode mixture layer on both sides of the aluminum foil.

[0158] <Preparation of Electrolyte Solution> An electrolyte solution was prepared by adding LiPF as a lithium salt to a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7. The concentration of LiPF in the non-aqueous electrolyte solution was 1.3 mol / L.

[0159] <Battery Fabrication> Lead tabs were attached to each electrode. Next, the positive electrode and negative electrode were spirally wound with a separator interposed between them so that the leads were located at the outermost periphery. In this way, an electrode group was fabricated. Next, the electrode group was inserted into an exterior body made of a laminate film with an aluminum foil barrier layer, and vacuum dried. Next, a nonaqueous electrolyte solution was injected into the exterior body, and the opening of the exterior body was sealed. In this way, a secondary battery was obtained.

[0160] <Evaluation> (Battery Capacity Degradation Rate) Each completed battery was placed in a 25°C environment and subjected to constant current charging at a current of 0.5 It until the voltage reached 4.2 V, and then constant voltage charging at a constant voltage of 4.2 V until the current reached 0.02 It. Subsequently, constant current discharging was performed at a current of 1.0 It until the voltage reached 2.5 V. The battery was then left for 20 minutes. This operation (charge / discharge cycle) was repeated 100 times. Charging and discharging were performed in a 25°C environment.

[0161] The discharge capacity at the initial discharge was defined as C0, and the discharge capacities after repeating the above charge-discharge cycle 50 times and 100 times were measured as C50 and C100, respectively. The change in capacity retention rate per cycle was calculated using the following formula and evaluated as the deterioration rate (%): (Deterioration rate (%)) = 100 × (C50 - C100) / (C0 × 50)

[0162] The results of the capacity retention rate, i.e., the deterioration rate, are shown in Table 1. Table 1 also shows the ratio, i.e., the deterioration rate ratio, when the deterioration rate of Comparative Example 1, which does not contain carbon black, is set as the reference (1.00).

[0163] (Viscosity of Negative Electrode Mixture Slurry) The viscosity of the negative electrode mixture slurry was measured one day after its preparation by the following method. The results are shown in Table 1. The viscosity of the slurry was measured using a B-type viscometer at 25°C. Table 1 also shows the ratio when the viscosity of the negative electrode mixture slurry of Comparative Example 1, which does not contain carbon black, is taken as the reference (1.00), i.e., the viscosity ratio of the slurry.

[0164] From the results shown in Table 1, the batteries of Examples 1 to 6, which had a CB / CNT ratio of 10 or more and less than 40, had a lower degradation rate and better battery characteristics than the battery of Comparative Example 1, which did not contain CB. Furthermore, in this case, the increase in slurry viscosity was also kept relatively small. In contrast, the batteries of Comparative Examples 2 to 4, which had a CB / CNT ratio of less than 10, had a degradation rate equal to or higher than that of Comparative Example 1, and the battery characteristics could not be maintained. Furthermore, the battery of Comparative Example 5, which had a CB / CNT ratio of 40 or more, had a large increase in slurry viscosity.

[0165]

[0166] In the following, Reference Examples 1 to 7, the relationship between the carbon nanotube content (mass % relative to the negative electrode active material) and the viscosity of the negative electrode mixture slurry was evaluated using single-walled carbon nanotubes. The negative electrode mixture slurries prepared in Reference Examples 1 to 7 had carbon nanotube contents varied from 0.03 mass % to 0.10 mass % relative to the negative electrode active material, compared to the negative electrode mixture slurry of Comparative Example 1, which did not contain carbon black. The viscosity of the slurry was measured using the same method as in the Examples and Comparative Examples. However, unlike the Examples and Comparative Examples, the viscosity of the Reference Example was measured on the slurry immediately after preparation. The results are shown in Table 2. Table 2 shows the viscosity ratio of the slurry, i.e., the ratio when the viscosity of the negative electrode mixture slurry of Reference Example 5 is taken as the reference (1.00).

[0167]

[0168] From the results shown in Table 2, it was found that the viscosity of the slurry became relatively high when the carbon nanotube content increased up to 0.10% by mass relative to the negative electrode active material, and therefore it was confirmed that the carbon nanotube content is preferably less than 0.10% by mass relative to the negative electrode active material.

[0169] In the following, Reference Examples 8 to 11, single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) were used as carbon nanotubes, and the differences in the resulting battery characteristics were evaluated. The single-walled carbon nanotubes used were the same as those used in the Examples and Comparative Examples. The multi-walled carbon nanotubes used had an average fiber diameter in the range of 9 nm to 10 nm. The battery used for the evaluation was the same as the battery of Comparative Example 1, except that the content of carbon nanotubes relative to the negative electrode active material was changed as shown in Table 3. Table 3 shows the ratio, i.e., the deterioration rate ratio, when the deterioration rate of Reference Example 8 is set as the reference (1.00). The viscosity of the negative electrode mixture slurry was also evaluated.

[0170]

[0171] As shown in Table 3, comparing the results of Reference Example 8 and Reference Example 9, it was difficult to maintain good battery characteristics with the same amount of multi-walled carbon nanotubes as with single-walled carbon nanotubes, and a higher content was necessary. However, increasing the amount in order to improve battery performance increased the viscosity of the slurry. Therefore, single-walled carbon nanotubes were preferred as the carbon nanotubes.

[0172] The technology of the present disclosure is useful for batteries such as lithium ion secondary batteries.

Claims

1. A negative electrode comprising a negative electrode mixture layer containing a negative electrode active material and a conductive additive, wherein the negative electrode active material includes first active material particles containing a carbon material as a main component and second active material particles containing silicon, the conductive additive includes carbon nanotubes and carbon black, the carbon nanotubes have an average fiber diameter of 0.5 nm or more and 6 nm or less and an average fiber length of 1.2 μm or more and 8 μm or less, and the mass ratio of the carbon black to the carbon nanotubes is 10 or more and less than 40.

2. The negative electrode according to claim 1, wherein the carbon nanotubes include single-walled carbon nanotubes.

3. The negative electrode according to claim 1, wherein the mass ratio of said carbon black to said carbon nanotubes is 15 or more and 30 or less.

4. The negative electrode according to claim 1, wherein the content of the carbon nanotubes in the negative electrode mixture layer is 0.01% by mass or more and less than 0.10% by mass with respect to the negative electrode active material.

5. The negative electrode according to claim 4, wherein the content of the carbon nanotubes in the negative electrode mixture layer is 0.01% by mass or more and 0.06% by mass or less relative to the negative electrode active material.

6. The negative electrode according to claim 1, wherein the content of the carbon black in the negative electrode mixture layer is 0.5% by mass or more and 2.5% by mass or less relative to the negative electrode active material.

7. The negative electrode according to claim 6, wherein the content of the carbon black in the negative electrode mixture layer is 0.5% by mass or more and 1.5% by mass or less relative to the negative electrode active material.

8. The negative electrode according to claim 1, wherein the second active material particles comprise composite particles including a matrix containing at least one element selected from the group consisting of carbon and oxygen, and a silicon phase dispersed in the matrix.

9. The negative electrode according to claim 1, wherein the second active material particles have an average particle size of 3 μm or more and 15 μm or less.

10. The negative electrode according to claim 1, wherein the content of the second active material particles in the negative electrode active material is 10% by mass or more and 60% by mass or less.

11. The negative electrode according to claim 10, wherein the content of the second active material particles in the negative electrode active material is 30 mass % or more and 60 mass % or less.

12. The negative electrode according to claim 1, wherein the first active material particles contain graphite as a main component.

13. The negative electrode according to claim 1, wherein the first active material particles have an average particle size of 5 μm or more and 30 μm or less.

14. The negative electrode according to claim 1, wherein the negative electrode mixture layer further contains a binder, and the binder contains at least one fluorine-free polymer material selected from the group consisting of carboxymethyl cellulose and alkali metal salts thereof, polyacrylic acid and alkali metal salts thereof, and copolymers containing butadiene units.

15. A battery comprising: a negative electrode according to any one of claims 1 to 14; a positive electrode; and an electrolyte.

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